Former Olympic champion and world record holder Liu, 29, had been left distraught in 2008 when he was forced to withdraw just moments before his opening heat in front of his home fans at the Beijing Games.

His return to the Olympic arena on Tuesday lasted just a split second when he ploughed into the first hurdle with his left leading leg and crashed heavily to the track.

Liu, who has been troubled by back and foot problems, eventually got up and hopped down the track to be embraced by his fellow competitors, winning warm applause from the stadium packed with 80,000 stunned spectators.

He was helped off the track by Britain's Andy Turner and Spain's Jackson Quinonez before being taken to the Olympic Stadium medical centre in a wheelchair.

American Aries Merritt, the leading hurdler this season, said he felt sorry for Liu.

"It was just terrible. For that to happen to one of the best hurdlers of all time is just a tragedy and I hope he's OK," said Merritt, whose achievement in running the fastest ever Olympic heat of 13.07sec was overshadowed by Liu's distress.

"He looked fine before the race, like nothing was wrong with him. He warmed up great. He always has a good warm-up and he was happy and so I don't think anything was wrong with him going into the race.

"I just think he made a small mistake."

Bolt, fresh from running the second-fastest time in history to retain his 100m title on Sunday, began his bid to keep his 200m title by winning his heat in 20.39sec.

"That was an easy run. I am enjoying it," said the Jamaican star. "This is my favourite event so I am looking forward to it.

"It's getting pretty chilly. They say it should be good on Thursday for the final. We'll see. I have heard that before."

World 100m champion, and Bolt's compatriot, Yohan Blake also made it through to the next round